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A Discrete Approach to Battery Charging for Cellular Phones

Guy Moxey and Michael Speed

AN817

INTRODUCTION

All portable cordless appliances must receive power from an
external source, whether it’s a wall cube adapter , car charger,
or docking station. This external source will then charge, in a
predetermined fashion, the equipment’s internal battery.

In the case of a portable phone, the power management
system will incorporate charging control circuitry to regulate
the voltage supplied to the battery from the external charger.
External charging equipment—whether wall cubes or
chargers that utilize car cigarette lighters—will supply a
continuous but unregulated voltage to the phone, typically

4.2 V for a single Lithium-ion (Li+) cell. A typical charging
design is explored in Appendix A.

Charge control for a Li+ cell is most commonly implemented
by a discrete MOSFET in series with a Schottky diode,
controlled via the onboard power management ASIC or
system microprocessor. Integration of these two discrete
power components into a single power package, such as the
ChipFETt, reduces size and simplifies the assembly.

Charger

+
–

LITTLE FOOT Plust

To this end, moving away from a separate Schottky diode and
MOSFET to the single package integration of both devices, as
in the Vishay Siliconix LITTLE FOOT Plus

TM

, may have
significant advantages. However, in an integrated package
both components operate in a highly dissipative manner,
making the choice of package a critical decision.

The LITTLE FOOT Plus Schottky diodes come in a variety of
packages, with a range of r
the performance of the charger as r

values. Just as important to

DS(on)

values are the

DS(on)

thermal ratings of the packages. From the table below we can
see the choices of R

values available in today’s

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industry-standard surface-mount packages.

TABLE 1.

LITTLE FOOT Plus PACKAGE OPTIONS

Device

SO-8 —Si4833DY 90

TSSOP-8 — Si6923DQ 115

TSOP-6 — Si3853DV 130

1206-8 ChipFET — Si5853DC 90

R

(_C/W) Typical

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POWER DISSIPATION ISSUES

To select the correct part in the smallest package, the power
dissipated by the two power devices must be examined. In the
case of the charger switch, there are two modes of operation
to consider.

PA

Battery

FIGURE 1. LITTLE FOOT Plust Schottky—The Integrated

Solution of MOSFET and Schottky in One Power
Package

+
–

Regardless of the charging device selected, the designer is
still bound by space, cost, and efficiency considerations.
There is therefore an obvious desire to increase levels of
integration and reduce the component count and board size.

Document Number: 71395
22-Jan-01

In the first phase of charging, constant current is used and the
MOSFET is operated in the linear mode. In this mode the
device is effectively a variable resistor used to regulate the
battery charging current.

Once the battery has charged to the predetermined 4.1-V
level, the system voltage loop will begin to reduce the charging
current in order to maintain the desired float voltage, hence the
constant-voltage mode. For constant-voltage operation, the
controller will terminate the MOSFET linear operation and
revert to a pulse width modulation (PWM) mode. The
MOSFET is driven as a fully-saturated (Ohmic) switch.

The Schottky diode is always required in series with the switch
to prevent reverse current flow through the MOSFET’s body
drain diode when the external power source is unplugged or
unpowered. Using separate MOSFETs and Schottkys rather
than an integrated package consumes valuable board space.

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Vishay Siliconix

CONSTANTĆCURRENT MODE

As the MOSFET operates in the linear mode during
constant-current charge control, the device losses can be
simply calculated from Ohm’s law:

P

LOSS

= (V

BAT(max)

– V

BAT(min)

) x I

OUT

As an effective linear regulator, the MOSFET functions in a
high-dissipation mode. In addition, the series Schottky diode
contributes to the overall power loss. Typical charging currents
range from 400 mA to 700 mA, with the Li+ battery voltage

varying from 4.1 V (fully charged) to 3.0 V (discharged)

. So at

a 500-mA charging current, the charging system for a Li+ cell
will have to dissipate:

P

MOSFET = (4.1 * 3.0)=) 0.5 = 0.55 W

LOSS

P

Schottky = VF IF = 0.48 0.5 = 0.24 W

LOSS

In total, under constant current charging the total discrete
power loss = 0.79 W

CONSTANTĆVOLTAGE MODE

For the constant-voltage portion of the charge cycle, the
MOSFET is fully saturated. Any consequent losses will be
minimal and, assuming the worst-case scenario of T
and V

of 2.5 V, can be found from:

GS

PD Conduction = (Irms)2 r

P

Switching = 1/2 VL (tr + tf) FS W

D

= (0.5)
= 0.058 W

2

0.232

DS(on)

W

of 150°C

J

with 60°C as the benchmark. Therefore, to optimize the power
dissipation to the smallest MOSFET package the following
thermal equation can be used:

PD = (T

J(max)

– T

amb

)/R

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Where:

PD = 0.79 W, T

Hence re-arranging for R

= 150_C and T

J(max)

= 114_C/W.

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amb

= 60_C

Good engineering practice allows a safety margin of 10% on
the T
R

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value, thus decreasing the allowable package

J(max)

to approximately 104°C/W.

From both the above calculation and Table 1, we can deduce
that the best-fit package for the MOSFET plus Schottky , while
still providing suitable power dissipation, are the SO-8
package and the recently introduced ChipFETt 1206
package.

However, moving upward in package footprint may not be an
option when heavily restrained by the device size. Therefore
the new ChipFET 1206 package offers a typical R

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of only
90°C/W, a footprint 40% smaller than a TSOP-6, and a thermal
performance previously possible only with the SO-8 package.

TSOP-6

Assuming switching of 100 Hz
t

and a tf value of 1 ms.

r

0.5 (4.1 0.5) (1 10 * 6 + 1 10 * 6) 100 = 2 mW

Therefore, the losses generated from the MOSFET under
constant-voltage operation are much less—approximately

7.6% —than the losses generated under constant-current
operation.

SMD PACKAGE THERMAL PERFORMANCE—

Can we dissipate the heat with LITTLE FOOT Plus?

Ambient temperatures usually quoted for component
calculation within a cell phone range between 50°C to 65°C,

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40% Smaller Footprint

1206-8 ChipFET

FIGURE 2.

Document Number: 71395

22-Jan-01